This invention relates to apparatus for providing high pressure and temperature for use in the formation of minerals and new materials.
Conventional high pressure units enable pressures of ˜15 GPa (using WC/Co anvils) and ˜100 GPa (for diamond anvils) in a working volume of ˜1 μm3, but with a limitation in temperature of about 2,000° C. However, if a reaction cell could be made with a larger working volume and even higher temperature capabilities, there is then the possibility of synthesizing diamonds directly from molten carbon in a relatively short time. Such synthesized diamond pieces will have fine grain size or single crystal structures, depending on the solidification rate.
Previous apparatus for achieving high pressures and temperatures may be found in: P. W. Bridgman Scientific American, Novermber 1955, p.42;. U.S. Pat. No. 2,941,248 to H. T. Hall “High temperature-high pressure apparatus”; and. U.S. Pat. No. 3,746,484 to L. F. Vereshchagin et al “Apparatus for achieving high pressure and high temperature”.
FIG. 1 shows schematically the key components of the high pressure/high temperature apparatus of the present invention, and the corresponding reaction cell (which holds the material to be processed), designed and implemented for the hot-pressing of carbon-based and other materials. The high pressure/high temperature apparatus consists of two profiled anvils 1 and three supporting steel rings 2-4 supporting each anvil. The anvils 1 squeeze a container 5 made of plastic stone and a reaction cell 6 that resides within the container. Cylindrical inserts 7 and 8 are disposed above and below profiled anvils 1 and are constructed from WC/6 wt % Co, which are supported by steel rings which are described in detail with below. The hardness of the supporting rings decreases from the center of the apparatus to the periphery. Reaction cell 6 consists of a graphite crucible that serves as a heater when electrical current is passed therethrough.
Supporting steel rings are used to increase the allowed load exerted on the anvils and inserts. In effect, they provide side-supporting pressure, which increases the effective fracture strength of the anvils under compression. A set of such supporting rings is needed, since the maximal supporting pressure that a multilayer cylinder can bear is twice the maximum pressure that can be achieved in a monolayer cylinder:Po(max)≈2σts/√{square root over (3)}  (1)where σts is the ultimate tensile strength of the steel. It is ˜2.0 GPa for hardened steel. This scheme permits a maximal working pressure in the RC (PWmax) that is higher than the compressive fracture strength of the anvils; however, this pressure is always less than the Vickers hardness (HV) of the anvils:σfs≦PWmax≦HV  (2)The maximal working volume (Vmax) that can be achieved under pressure depends on σfs, maximal loading force (Fmax) of press, size of frame window (aj) and size of anvils used (Va):Vmax=Vmax(σfs, Fmax, aj, Va)  (3)According to theory, the fracture compressive strength of a brittle material is inversely proportional to the sample volume:σfs=ησcsVa−γ  (4)where η=η0V0aγ is constant, η0 is dimensional constant that is typical of a given material, V0a is the volume of a standard sample for measuring compressive strength (σcs) and exponent γ is a typical value for a given material (γ˜ 1/15 for regular WC/Co). The values of σcs and HV in formula (2) are also interrelated. The HVst is ˜2.5σcsst for hardened steel that has some plasticity. The HVcer is ˜7σoscer for brittle rocks, stones and ceramics. The HVcom is about from 3 to 5σcsst for composite materials with brittle skeleton and plastic matrix, such as materials of the WC/Co type. The degree of sensitivity of the compressive fracture strength on sample volume depends on porosity, crystallite size, and value of side supporting pressure (Pss):γ=γ(ρA, d, Pss)  (5)where ρA is apparent density, d is typical size of crystallites.
The high pressure/high temperature apparatus of the present invention enables a maximum possible static pressure over the range 1-100 GPa Hereafter, we will call the range 1-10 GPa “very high pressure” and the range 10-100 GPa “super high pressure”. Even higher pressures in large volume can, in principle, be achieved with the help of dynamic methods. We will call this pressure range (P>100 GPa) “ultra high pressure”.
Let us now consider how to achieve very high temperatures in the high pressure/high temperature apparatus. The necessary high temperature is best realized by passing an electric current directly through the graphite container. The thermal regime of the reaction cell and its container may be computed from the following equation:Wdt=∫cρ·dT·dV+(λ·gradT·dS)dt  (6)where W=qdV is power, q is power emitted in unit volume, c is specific heat capacity, ρ is density, λ is thermal conductivity. This equation in the static state may be represented as:div(λ·gradT)=0  (7)
An approximate solution of equation (6) for spherical thermal conductivity provides an opportunity to determine the thickness of thermal insulation and the relaxation time that is needed for the reaction cell to respond to a power change and to achieve steady state:                                                         T              =                                                T                  max                                ⁡                                  (                                      1                    -                                          e                                              -                                                  i                          τ                                                                                                      )                                                      ⁢                                                  ⁢                                                            where                  ⁢                                                                           ⁢                                      T                    max                                                  =                                  W                  /                  v                                            ;                                                           ⁢                              τ                =                                  η                  /                  v                                            ;                        ⁢                                                  ⁢            v            =                          ∫                                                div                  ⁡                                      (                                          λ                      ⁢                                                                                           ⁢                      grad                      ⁢                                                                                           ⁢                                              ψ                        ⁡                                                  (                          r                          )                                                                                      )                                                  ⁢                                  ⅆ                  V                                                              ;                ⁢                                  ⁢                              η            =                          ∫                                                c                  ·                                      ρψ                    ⁡                                          (                      r                      )                                                                      ⁢                                  ⅆ                  V                                                              ;                                    (        8        )            and ψ(r) is a function typical of a specific container. If the energy released is not uniform over the entire volume of the sample, some interval of time will be necessary to heating the center of the sample to a temperature close to that of the heater, Tmax. A typical relaxation time (τ0) depends on the materials properties and size of samples:                               τ          0                =                              1            3                    ⁢                                                    c                0                            ⁢                              ρ                0                                                    λ              0                                ⁢                      r            0            2                                              (        9        )            where c0, ρ0, λ0 are heat capacity, density and thermal conductivity of sample and r0 is radius of sample.
A temperature range of 100-2000° C. is achievable using graphite heaters. In the temperature range 2000-4000° C., the carbon does not melt, but reacts with all elements and compounds, with the exception of inert gases. The very high temperature range, up to 4000-5000° C., is difficult to obtain, especially under high pressure, because the efficiency of thermal insulation is limited.
Thermal flow from the heater is proportional to the first power of temperature, but thermal flow by radiation is proportional to the fourth power of temperature:dEe=σSTβT(r)αT(r)T4dSdt  (10)where αT is blackness, σST is Stefan-Boltzman constant, βT is integral coefficient of reflection of electromagnetic waves from the surface. The energy dEe added to the right side of eq. (6) increases the heat transfer at very high temperature and leads to a situation where a small increase in temperature demands a large increment in heater power.
The apparatus to be described below permits the achievement of static super high pressure in combination with static very high temperature.